Monocrystalline silicon wafers are of considerable significance in the production of semiconductive circuit elements for present-day and future high technology. The production of such silicon wafers generally requires the lowering of a high-purity monocrystalline silicon seed into a melt of elemental silicon contained in a quartz crucible or vessel which must, because of its sensitivity to the temperature at which the melt must be held, generally be supported in a graphite or carbon outer vessel. Usually the crucible and the seed, as the latter is retracted, are relatively rotated and controlled cooling is effected so that the bar which is drawn from the melt or grown on the seed has a monocrystalline structure.
The bar is then cut up transversely to wafers of the desired thickness. The process is carried out under a controlled atmosphere or vacuum to minimize contamination of the bar, the silicon melt and the wafers.
In general, the melt in the quartz crucible is maintained at the requisite temperature for drawing the silicon bar, say, 1400.degree. to 1450.degree. C. by induction heating via an induction-heating coil located outside the quartz crucible and surrounding the supporting carbon vessel. Since the heat is applied from the exterior inwardly, the quartz wall and the outer portions of the malt may be at a somewhat higher temperature than the silicon melt or points within the body of the melt.
At these high temperatures, which are close to the softening point of the melt, material from the crucible tends to diffuse into the melt, the crucible has a tendency to soften under the mechanical stresses which arise at the high temperatures and because of the fact that these high temperatures must be maintained for the entire bar-drawing process. Such periods may be tens of hours in duration.
As a consequence, deformation and deterioration of the crucibles is a significant problem and one which can be attacked by a method as described in my commonly assigned application Ser. No. 614,434, filed May 25, 1984 and entitled METHOD OF COATING CERAMICS AND QUARTZ CRUCIBLES WITH MATERIAL ELECTRICALLY TRANSFORMED INTO A VAPOR PHASE. That application is a continuation-in-part of my copending application Ser. No. 494,302 filed May 13, 1983 as a continuation-in-part of Ser. No. 358,186 of Mar. 15, 1982 (U.S. Pat. No. 4,438,183), in turn a continuation-in-part of Ser. No. 237,670 filed Feb. 24, 1981 (U.S. Pat. No. 4,351,855), incorporating the subject matter of Disclosure Documents Nos. 078,337, 078,334 and 078,329, all of Feb. 26, 1979 and Disclosure Document No. 082,283 of July 5, 1979. In that system, compounds of silicon formed in situ such as silicon carbide and silicon nitride can be deposited on the inner surface of the quartz crucible as a protective coating. Such coating, of course, can only be of limited effectiveness because the softening effect is not eliminated although the coating provides a significant reduction in the migration of contaminants from the crucible into the melt.